Pressure compensator for speaker cabinets

ABSTRACT

A pressure compensator for speaker cabinets is provided which includes a flexible rubber diaphragm attached to the rear of a speaker cabinet covering a plurality of vent holes and which includes an adjustable rod engaged in a plastic dome within a sponge rubber ball at the center of the diaphragm which extends the diaphragm so as to form a logarithmic profile to substantially reduce the back wave. A second diaphragm may also be employed spaced from the first mentioned diaphragm for greater suppression.

United States Patent Liebscher 1 June 6, 1972 [54] PRESSURE COMPENSATOR FOR SPEAKER CABINETS [72] Inventor: Arthur Liebscher, c/o Electronics, Inc., In-

dustrial Park, Willow Grove, Pa. 19090 [22] Filed: Sept. 15,1970

[2]] Appl.No.: 72,463

52 us. Cl ..l8ll3l a [58] FieldofSearch ..181/31 B,3l R

[56] I References Cited v UNITED STATES PATENTS 2,713,396 7/1955 Tavares ..l8 l/3l B 3,554,313 1/l97l Young ..l8l/3l B Primary Examiner-Stephen J. Tomsky Attorney-Zachary T. Wobensmith, 2nd

[57] ABSTRACT A pressure compensator for speaker cabinets is provided which includes a flexible rubber diaphragm attached to the rear of a speaker cabinet covering a plurality of vent holes and which includes an adjustable rod engaged in a plastic dome within a sponge rubber ball at the center of the diaphragm which extends the diaphragm so as to form a logarithmic profile to substantially reduce the back wave. A second diaphragm may also be employed spaced from the first mentioned diaphragm for greater suppression.

8 Claims, 5 Drawing figures P ATENTED UN 6W2 3.667. 568 SHEET 10F 2 //v VENTOI? ARTHUR L/EB s CHE/1 ATTORNEY PATENTEDJUH 6 m2 3, 667, 568

saw 2 OF 2 INVENTOR ARTHUR L/EBSCHER ATTORNEY PRESSURE COMPENSATOR FOR SPEAKER CAEINETS BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to pressure compensators for speaker cabinets of the type which contain or control rear sound wave effects and permit rearward venting of phase delayed sounds.

2. Description of the Prior Art The growing use of high fidelity sound reproduction systems has resulted in demands for better speaker systems. In the common type of speaker equipped with a cone, a back wave is produced which is l80 out of phase with the front wave and which must be substantially suppressed.

Usually extensive baffling, large folded horns or sealing of the cabinet is necessary. None of these measures are wholly satisfactory, particularly with the small speakers in demand today. The apparatus of the invention is small in size yet effectively delays backwave emission from the rear of thecabinet without appreciably affecting or canceling the front wave.

SUMMARY OF THE INVENTION In accordance with the invention, a pressure compensator for loudspeaker cabinets is provided having a'flexible rubber or like diaphragm, and an adjustable cushioned center rod support to produce a logarithmic non-linear profile for delaying and reducing backwave effect, the non-linear profile being non-resonant at all bass and low midrange frequencies. A second diaphragm can also be employed in spaced relation to the first diaphragm for greater suppression.

It is an object of this invention to provide a non-linear variable spring constant pressure compensator device automatically adjustable to low frequency sound volume and resultant air compression.

It is a further object of this invention to provide a pressure compensator which forms a relatively small sealed enclosure wherein the back wave of a loud speaker cone is contained, except for the passive reaction of a phase-delayed waveform which is radiated through a venting port. It is a further object of this invention to provide a pressure compensator which is manually adjustable to obtain the optimum equalization of front and backwaves and particularly at the frequency of the system resonance.

Other objects and advantageous features of the invention will be apparent from the description and claims.

BRIEF DESCRIPTION OF T I'IE DRAWINGS the line 4-4 of FIG. 3;

FIG. 5 is a vertical sectional view, of a modified form of the invention. i

It should, of course, be understood that the description and drawings herein are illustrative merely and that various modifications and changes can be made in the structure disclosed without departing from the spirit of the invention.

Like numerals refer to like parts throughout the several views.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now more particularly to FIGS. 1 to 4 of the drawings, a loudspeaker is there illustrated of the character disclosed in my pending applications, Ser. Nos. 864,320, now abandoned and 864,322, now abandoned filed Oct. 7, 1969,

and Ser. No. 870,032, filed Oct. 28, 1969, and mounted within a loudspeaker cabinet 11 of any desired type preferably having a natural period of vibration subdued to a non-critical response level and preferably of wood.

The loudspeaker 10 illustrated by way of example, includes a rigid front ring 12 connected by frame bars 13 to a rear frame plate 14 and held by bolts 14a, the ring 12 being mounted to the front wall 15 of cabinet 1 1 by screws 16, and with one pole piece of a permanent magnet structure 18 secured thereto, such as by screws 19. The magnet assembly also includes a core 20, spaced to provide a flux gap 21.

A surround 22 may be provided in the form of a flat ring of sliced silicone rubber sponge held in place by a mounting ring l7, and screws 17a and the surround 22 engages the periphery 23 of the cone 24.

A voice coil 25 is provided extending in the gap 21 along the core 20 and secured to the diaphragm 24.

Two spaced coaxial supports 27 and 28 are provided of any desired flexible and resilient material, such as paper, impregnated cloth or synthetic plastic, with concentric corrugations 29. The supports 27 and 28 at their outer periphery are spaced by a frame ring 30. The ring 30 is held in assembled relation to the rear plate 14 by bolts 32 and compression spacer washers 34 interposed between the frame ring 30 and the frame plate 14. Radial screws 31 carried in the frame bars 13 engaging the periphery of the ring 30, provide for centering adjustment of the frame ring 30 and parts carried thereby.

The support 27 at its inner margin is secured to the coil 25 preferably by a suitable adhesive, and the support 28 at its inner margin, is preferably similarly secured to the coil 25 and to the cone 24 by a suitable adhesive, such as an epoxy resin.

A diffuser 35 is provided detachably secured to the core 20 by stud 26. The speaker just described forms no part of the-invention and is merely illustrative of one type of speaker with which the pressure compensator may be used.

In the reproduction of sound it is common practice to use a loudspeaker equipped with a cone, the reciprocal motion and vibration of which, produces sound waves in air. The wave generated by the front of the cone is the same as that generated by the back except that the two are 180 out of phase.

This fact makes no difference in the emitted wave of high audio frequencies where the wave length is shorter than the cone diameter. Conversely, wave lengths greater than the cone diameter will merge beyond the cone where, due to the 180 phase difference, they neutralize in the air with subsequent loss of sound.

By extending the surface of the cone with a flexible surround and continuing'with a fixed bafile such as the cabinet 1l,the same acts as an air separator and consequently a front and back sound wave separator. The size of the baffle must be extended as the low frequency wave length is increased to a limit where the lowest desired frequency is accommodated.

Because a technically correct bafile is too large for typical application, other means have been devised to extend air separators beyond the size of a speaker. Common methods of isolating or eliminating the rear wave effect are extensive spring constant of the system which consequently raises the square root of the spring constant (k) such limitations are responsible for bass cut off.

Because of the shorter internal resonant dimensions, however, a small cabinet has an advantage over larger cabinets in the absence of bass redundancy or cabinet boom.

Such a small cabinet, without bass reverberation characteristics, when matched to a speaker having fast transient response, will vastly improve the attack and decay transient response of the speaker system.

These attributes are not diminished when has resonance of the cabinet-speaker combination can be adjusted to alter the spring constant and to alleviate internal resonance pressure.

The pressure compensator 50 is mounted on a rear wall 51 of cabinet 1 l which rear wall 51 has a plurality of spaced holes 52 therethrough and centrally located. The compensator 50 includes a diaphragm 53 which is illustrated as composed of sponge rubber including natural rubber, synthetic rubber and silicone rubber, but which may be fabricated of any suitable material which acts in the desired manner.

The diaphragm 53 is retained in therear wall 51 by two rings 55 which secure the edge of the diaphragm 53 between them and have screws 56 therethrough engaged with the rear wall.

Another hole 57 is provided in rear wall 51 through which a threaded rod 60 extends and a tee nut 65 is embedded in the rear wall 51 and threadably engaged with rod 60. The rod 60 has a nut 61 on the exterior bearing against a washer 62 which engages the rear wall, both the nut 61 and washer 62 may be contained in a recess 63. The rod 60 has a slot 64 in the rear end to permit of adjusting the length of rod in front of wall 51. The nut 61 serves to prevent the rod from independent move ment. The rod 60 at its forward end has a plastic dome 66 secured thereto in any desired manner such as by epoxy cement. The plastic dome 66 may be encased in a ball 67 of sponge rubber or other suitable cushioning material.

The diaphragm 53 serves to form a continuous baffle and to seal the cabinet. The support of the diaphragm 53 with its edge retention on the rear wall 51 by the rings 55 and with the engagement of the diaphragm 53 by the ball 67 on the plastic dome 66, which dome is positioned by the rod 60, causes the diaphragm 53 to assume a shape which is logarthmic or exponential in cross section or profile. The non-linearity of that shape imparts to the diaphragm the characteristic of being non-resonant at all bass and low midrange frequencies.

Adjustment of the profile or cross section and consequent optimum spring constant is obtained by turning the rod 60 while listening to speaker response to obtain the optimum equalization of front and back waves and particularly at the frequency of the system resonance, after which the rod can be locked in place by tightening the nut 61 against washer 62.

The design includes means to facilitate adjustment without twisting the diaphragm 53 around the rod 60. This is accomplished by a lubricated engagement of the smooth plastic dome 66 with the hollowed sponge rubber ball 67. The dome 66 being cemented to the rod 60 turns with it, while the ball 67 through pressure contact with the diaphragm 53, overrides the minimal torque of the lubricated interface.

' The ball 67 also serves to relieve stress in the diaphragm 53 at the apex of its profile, thus minimizing the possibility of eventual rupture.

In functional operation, the contoured sponge rubber diaphragm 53 is sensitive to air pressures generated by the backwave of the speaker and will stretch, to accommodate increased or decreased pressure. In so doing, a broad differential spring constant replaces the heretofore fixed resonant frequency spring constant.

Through passive reaction, pressures affecting the inside of the diaphragm 53 are reproduced in delayed phase on the outside. This fact, plus the spring constant of the diaphragm 53 results in rearward pressure variations which are phasedelayed emitted with respect to the backwave from the speaker cone and therefore, substantially reduce the effect of backwave cancellation.

Referring now more particularly in FIG. 5 of the drawings, a

modified form of the invention is there shown with which a greater suppression of backwave effect is attained.

In FIG. 5 the construction is similar to that of FIG. 3 but a second diaphragm 53a is provided of the same or similar material to that of the diaphragm 53. The diaphragm 53a is clamped at its periphery by an additional clamping plate 550, held by screws 56 parallel to clamping plate 55.

The diaphragm 55a, at its center, engages a second hollow sponge rubber hemisphere 67a which engages over the portion of diaphragm 53 which engages the ball 67.

The spring action of the diaphragms 53 and 53a is thus assured so that there is no slack portion of either diaphragm which would interfere with the desired flexing action.

I claim: 1. In a loudspeaker cabinet enclosure having a front wall with a speaker mounted thereon having a movable diaphragm separating front and back waves,

said enclosure having a rear wall with at least one opening therethrough, and 7 pressure compensator means for said enclosure mounted on said rear wall in sealing relation to said opening equalizing said front and back waves at system resonance,

said pressure compensator means comprising an elastic diaphragm in closing relation to said opening,

and

a fixedly positioned member engaging the central portion of said diaphragm to hold said diaphragm in a shape free from resonance at bass and low midrange frequencies.

2. The combination defined in claim 1 in which said shape is exponential in cross section 3. The combination defined in claim 1 in which said member includes a rod mounted on said rear wall.

4. The combination defined in claim 3 in which said rod is manually adjustable.

5. The combination defined in claim 3 in which said rod is a threaded rod adjustably mounted on said rear wall.

6. The combination defined in claim 1 in which said member includes a ball in engagement with said diaphragm.

7. The combination defined in claim 6 in which a plastic dome is provided on which said resilient ball is carried and said dome is carried by a rod adjustably mounted in said rear wall.

8. The combination defined in claim 1 in which a second elastic diaphragm is provided centrally engaged in spaced relation to the first diaphragm and with its outer margins clamped in spaced relation to the outer margins of the first mentioned diaphragm. 

1. In a loudspeaker cabinet enclosure having a front wall with a speaker mounted thereon having a movable diaphragm separating front and back waves, said enclosure having a rear wall with at least one opening therethrough, and pressure compensator means for said enclosure mounted on said rear wall in sealing relation to said opening equalizing said front and back waves at system resonance, said pressure compensator means comprising an elastic diaphragm in closing relation to said opening, and a fixedly positioned member engaging the central portion of said diaphragm to hold said diaphragm in a shape free from resonance at bass and low midrange frequencies.
 2. The combination defined in claim 1 in which said shape is exponential in cross section
 3. The combination defined in claim 1 in which said member includes a rod mounted on said rear wall.
 4. The combination defined in claim 3 in which said rod is manually adjustable.
 5. The combination defined in claim 3 in which said rod is a threaded rod adjustably mounted on said rear wall.
 6. The combination defined in claim 1 in which said member includes a ball in engagement with said diaphragm.
 7. The combination defined in claim 6 in which a plastic dome is provided on which said resilient ball is carried and said dome is carried by a rod adjustably mounted in said rear wall.
 8. The combination defined in claim 1 in which a second elastic diaphragm is provided centrally engaged in spaced relation to the first diaphragm and with its outer margins clamped in spaced relation to the outer margins of the first mentioned diaphragm. 